Esterase enzymes in blood are the major metabolic pathway for cocaine in humans. There is population variability in enzyme activity, some of which is genetically based, yet no published studies to date have evaluated enzyme activity in cocaine abusers or the correlation between enzyme activity and response to cocaine. This study, done in collaboration with Dr. Raymond Woosley, Department of Pharmacology, Georgetown University, measures plasma and RBC esterase activity in cocaine abusing research subjects at the ARC to determine the influence of this metabolic parameter on response to cocaine and on treatment outcome.